Focused ion beam, or “FIB,” systems, represent a type of charged particle beam system that has become an important tool for integrated circuit (IC) manufacturers in bringing to the electronics marketplace highly reliable ICs that expand the bounds of capacity and performance.
To explain, a newly-designed custom IC typically is fabricated over a process of several weeks, involving preparation of silicon substrate wafers, generation of masks, doping of the silicon substrate, deposition of metal layers, and so on. Only after performing a long series of such steps is an actual IC produced for testing purposes.
Unfortunately, a new integrated circuit of any complexity rarely works as expected when first fabricated. Normally, some defects in the operation of the IC are discovered during testing. Also, some functions of the IC may operate properly under limited conditions, but fail when operated across a full range of temperature and voltage ranges in which the IC is expected to perform.
Once the IC has been tested, the designer may then make changes to the design, initiate the manufacture of a second prototype IC via the lengthy process described above, and then test the new IC once again. However, no guarantee exists that the design changes will correct the problems previously encountered, or that all of the problems in the previous version of the IC have been discovered.
To address these issues, IC manufacturers sometimes employ a FIB system to edit the prototype IC, thereby altering the connections between the transistors and other electronic structures embedded in the silicon layer of the device. By removing various sections of metal and other material to eliminate unwanted electrical connections, and by possibly adding metal segments to produce previously nonexistent circuit links, the design of the IC may be changed in a matter of a few hours or days instead of weeks, thereby greatly contracting the multiple-pass design-and-test cycle often employed to produce a defect-free IC.
Additionally, a FIB system is often used to generate an image of the integrated circuit. In conjunction with a CAD (Computer-Aided Design) layout of the various layers that make up the IC, a video image may be generated by the FIB system to aid in locating the specific areas of the IC that are to be edited. Use of such an image helps prevent editing mistakes which would normally render an IC useless.
With ICs generally becoming increasingly complex, and the number of transistors available within an IC continually growing, the use of a FIB system for imaging and editing an IC for altering the circuit design of the IC without engaging in multiple passes of the normal design-and-test cycle is well-known to improve overall time-to-market.
Generally speaking, FIB systems employ a high-powered, focused beam of metal ions, such as gallium, impinging the surface of an IC to provide the aforementioned imaging and editing capabilities. The width of the beam is typically on the order of a few nanometers so that extremely small features of the IC may be edited without unintended damage to the surrounding portions of the circuit. The beam is normally moved by electrostatic deflection of the beam by electronic control over the surface of the IC in a raster fashion. In other words, the beam is deflected across the surface in a series of closely-spaced horizontal parallel line segments, called “scan lines,” which together typically cover a rectangular area of the IC, called a “scan frame” or “edit box.” Along each scan line, the ion beam remains momentarily focused on each of a series of locations, called “pixels.” Accordingly, the length of time the ion beam remains focused on a particular pixel, called the “pixel dwell time,” determines the amount of etching or deposition that occurs on each impacted pixel of the DUT. The analog deflection of the beam, which ultimately determines the area to be scanned, the speed of the scan, the pixel dwell time, and so on, is typically controlled digitally by a microprocessor, microcontroller, digital signal processor (DSP), or similar software-driven algorithmic processor.
While the use of a microprocessor, DSP or similar device allows programmable, direct control of the focused ion beam in a FIB system, the potential speed and functionality of FIB system operation typically is restricted compared to a more hardware-intensive solution, thus possibly limiting the overall throughput and functional capability of the FIB system to edit an IC. For example, the pixel dwell time for current FIB systems typically is maintained constant throughout an entire scanning frame, and is normally determined by the size of the frame (or edit box) and the current of the ion beam. Modification of the pixel dwell time on a per-line or per-pixel basis, given typical dwell times on the order of 50–100 nsec, generally is not possible for even the fastest processors or DSPs, given the amount of overhead incurred in terms of the number of clock cycles required to execute each instruction of the software controlling the ion beam. In most FIB systems, limits on the number and length of the scan lines, the amount of pixel “overlap,” and other scanning parameters are also prevalent.
Accordingly, a need exists in the art for an improved charged particle beam scanning control method and apparatus.